1. Technical Field
The present invention relates to a correlation calculation unit calculating a correlation between an OFDM signal having effective symbol intervals and guard intervals obtained by copying a part of a signal in one effective symbol interval, and a delay signal delayed from the OFDM signal.
2. Related Art
In recent years, an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme has been used as a modulation scheme for digital terrestrial broadcasting or the like. In the OFDM scheme, a symbol is transmitted using a plurality of sub-carriers differing in central frequency. “Symbol” as used herein means a set of data transmitted per modulation.
One symbol interval includes an effective symbol interval and a guard interval. As shown in FIG. 9, in the OFDM scheme, the influence of multipath interference is suppressed by copying a part of a signal in the effective symbol interval to be actually demodulated and inserting the copied part, as a repeating waveform, between effective symbol signals in the effective symbol intervals. The interval of this copied waveform corresponds to a guard interval.
To demodulate the OFDM signal, one received OFDM signal is converted into a digital signal by an A/D converter, the guard interval is eliminated and the effective symbol signal is extracted from the digital signal, and the effective symbol signal is demodulated by an FFT (Fast Fourier Transformer). Specifically, as shown in FIG. 10, a correlation value between the received OFDM signal and a signal delayed from the OFDM signal by the length of the effective symbol interval is calculated. A maximum value of an integrated correlation value is extracted, the guard interval is eliminated and the effective symbol interval is extracted from the OFDM signal based on the timing of the maximum value, and the extracted effective symbol interval is demodulated by FFT. This conventional technique is disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 11-163824 and 2000-059332.
The conventional OFDM has, however, the following problems. If received (reception) power is low, the influence of fading or multipath is great, or a narrowband noise signal enters a reception band, then the correlation value may become small and offset or lose the timing of the maximum value, resulting in deterioration in reception performance.
A technique for calculating a correlation value by multiplying each of a real image component and a virtual image component of a received signal by polarity information to make a complex calculation is also known as disclosed in, for example, JP-A No. 2000-295194. This correlation calculation technique has, however, the following problems. The correlation value is calculated using a polarity-inverted signal directly. Due to this, a noise component is added to the correlation value directly and the influence of noise cannot be suppressed. Further, circuit size cannot be reduced.